|execute calculator         |
|v1|v2|operator     |result?|
|30|4 |+            |34     |
|30|4 |-            |26     |
|5 |  |!            |120    |
|4 |6 |*            |24     |
|8 |2 |/            |4      |
|1 |  |-            |-1     |
|  |  |+            |0      |
|8 |0 |/            |<error>|
|  |  |$%^unknown*&^|<error>|
